Resize the photo
Open the photo in your favorite photo editor. (I use Adobe Photoshop Elements, but the steps are much the same in any photo-editing program.) Choose Image, Resize, Image Size. In the Document Size section of the dialog box, resize the photo so that it fits on the front of the card, with a small margin around the edges.
Our card will be 4 inches tall and 3 inches wide, so let's enter a height of 3.5 inches and let the width set itself automatically to keep the photo's dimensions proportionate. (Make sure that the Constrain Proportions check box is checked.) Those specs will ensure that the picture a little smaller than the card, producing a snazzy blank border around the image. Click OK.